The camlp4 API permits, via "Grammar.extend" to create empty precedence level.
This is usefule since you can assign names to them and make assumptions on their
relative order in future extensions of the grammar.

If an empty level gets extended with a rule, the obtained level then contains
only one rule. When that rule is removed using "Gramext.delete_rule" the entire
precedence level is removed, making further extension of it failing with
something like: